Make sure the encoding is on mbstring's list of supported encodings b…

…efore trying to convert.

Thanks to mattfaulds on GitHub, and solarissmoke on WordPress Trac for patches.

simplepie.inc

@@ -9461,7 +9461,7 @@ class SimplePie_Misc
 			return SimplePie_Misc::windows_1252_to_utf8($data);
 		}
 		// This is second, as behaviour of this varies only with PHP version (the middle part of this expression checks the encoding is supported).
-		elseif (function_exists('mb_convert_encoding') && @mb_convert_encoding("\x80", 'UTF-16BE', $input) !== "\x00\x80" && ($return = @mb_convert_encoding($data, $output, $input)))
+		elseif (function_exists('mb_convert_encoding') && @mb_convert_encoding("\x80", 'UTF-16BE', $input) !== "\x00\x80" && in_array($input, mb_list_encodings()) && ($return = @mb_convert_encoding($data, $output, $input)))
 		{
 			return $return;
 		}